What a lot of posters have already said, go to a good independent bed retailer, they are much more knowledgeable than the ‘Chains’.
We did this earlier this year, as our old comfortable mattress was now not so comfy, and about 11 years old.
The make we bought was Gainsborough, and it was expensive, but think how long you spend in bed.
At first I used a mattress topper ( memory foam) on it, but it was affecting my hip ( memory foam is hard to turn over on) and think I was getting stuck in one sleeping position for too long.So, have put away the topper, and now sleep better.
The mattress is designated as medium firm.
All mattresses will sag in time, and need turning regularly ( unless you buy a non turn one, of course.)
Oh for the days when I could, and did, sleep on anything, especially on many camping holidays with the children.